For royal icing:
Mix 2 cups of powdered sugar, 1½ Tablespoons meringue powder, and 3 Tablespoons of warm water in a stand mixer for 3 to 4 minutes. If you pull the beater out of the icing, it should be thick enough that the dripping icing is very slow to meld back into the icing in the bowl but not so thick that it is a frosting consistency. If it is too thin, add some more powdered sugar. If it is too thick, add a little water. You'll want the icing to be a bit thicker than what you would use to decorate cookies.